Torque proportioning differentials develop a frictional resistance to relative rotation (i.e., differentiation) between output shafts substantially proportional to the total amount of torque transmitted to the output shafts. The resistance to the relative rotation of the output shafts is reflected as a torque difference between the output shafts. The torque difference varies substantially linearly with the sum of torque of the output shafts as expressed in the following equation: EQU T.sub.d =k T.sub.s
where "T.sub.d " is the torque difference between output shafts, "T.sub.s " is the torque sum of the same shafts, and "k" is a proportionality constant.
However, this torque proportioning characteristic is more commonly expressed as a "bias ratio", which is a ratio of the respective amounts of torque in the two output shafts. The bias ratio "B" can also be expressed in terms of the torque difference "T.sub.d " and torque sum "T.sub.s " as follows: ##EQU1##
Torque proportioning differentials generally exhibit a relatively constant bias ratio, which is selected to meet particular traction objectives. For example, a torque proportioning differential with a bias ratio of 3 to 1 can deliver a total amount of drive torque to a pair of drive wheels equal to four times the amount of drive torque exerted by the drive wheel having the least traction.
However, the same distribution of drive torque is required to permit any relative rotation between drive axles. For example, an increased amount of the total drive torque is delivered to the slower rotating drive wheel regardless of traction conditions. This increased amount of torque is equal to the torque difference that opposes relative rotation between drive axles.
Any torque difference "T.sub.d " between front or rear drive wheels contributes to a vehicle yaw moment that could influence vehicle lateral stability. While differentiating under power, the torque difference between output shafts required to permit differentiation produces a vehicle understeer moment. However, if the required torque difference cannot be sustained by available traction, differentiation ceases and the remaining torque difference can produce either an understeer or an oversteer moment. Torque differences between front drive wheels can also produce so-called "torque steer" moments that oppose steering efforts.